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Abrasive machining of porcelain and zirconia with a dental handpiece
Wear 255 (2003) 975–989
Abrasive machining of porcelain and zirconia with a dental handpiece L. Yin1 , S. Jahanmir∗ , L.K. Ives National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899-8521, USA
Abstract The machining characteristics and material removal mechanisms of two dental ceramics—feldspathic porcelain and yttria-stabilized tetragonal zirconia—were investigated using a dental handpiece and diamond burs with different grit sizes. The material removal rates were measured as a function of total machining time using a constant load of 2 N on the bur, consistent with clinical cutting conditions. As the diamond grit size was increased from ultrafine (UF) (10 m) to fine (F) (41 m) and coarse (C) (172 m), the removal rate and the resulting surface roughness for each material increased substantially. The mechanisms of material removal determined through microscopic examination of the machined surfaces and the machining debris on the burs were found to consist of a combination of ductile and brittle-type chip formation processes. The occurrence of brittle fracture increased as the diamond grit size was increased. While the material removal process in porcelain was dominated by brittle fracture, zirconia was primarily subjected to ductile cutting. Four wear processes were identified on the burs in prolonged cutting tests: grit microfracture, grit pullout, wear flat generation, and matrix abrasion. The results demonstrated that while the material removal rate for the zirconia evaluated in this study was lower than those for porcelain and many other dental ceramics, the zirconia could be machined under clinical conditions with no edge chipping damage. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Dental restorations; Diamond burs; Dental handpiece; Yttria-stabilized tetragonal zirconia; Felspathic porcelain; Material removal mechanism; Tool wear; Surface integrity
1. Introduction Abrasive machining is routinely used in dentistry for the preparation of teeth for filling, crowns, and bridgework, and for making final fit adjustments on dental restorations [1]. In these machining procedures, a high-speed, air-driven dental handpiece with a diamond bur is often used. In spite of the importance of diamond machining in dentistry, there are only a few published studies on the subject [2–8]. Recent studies have provided data on the machining characteristics of tooth enamel [4], machinable glass-ceramics [6], and glass-infiltrated alumina [7] using a dental handpiece and diamond burs. The purpose of the present study is to investigate the abrasive machining characteristics of a commonly used dental porcelain and a new tetragonal zirconia. In addition to handpiece machining performed by the dentist, abrasive machining is used to prepare ceramic restorations in the dental laboratory using computer-aided ∗ Corresponding author. Present address: MiTiHeart Corporation, P.O. Box 83610, Gaithersburg, MD 20883, USA. Fax: +1-301-869-9724. E-mail address: [email protected] (S. Jahanmir). 1 Present address: Gintic Institute of Manufacturing Technology, Singapore 638075, Singapore.
design and computer-aided manufacturing (CAD/CAM) systems [9,10]. The desired shapes for crowns, onlays, or inlays are obtained fairly rapidly using either a milling cutter or an abrasive tool depending on the properties of the workpiece material. The data obtained in abrasive machining of porcelain and zirconia with the dental handpiece can also be used as a guide for CAD/CAM preparation of dental restorations with these ceramics, where similar machining conditions apply. Porcelains have been widely used for dental restorations as veneers mainly because of their excellent esthetics [11–13]. More recently, modified feldspathic porcelains containing crystalline reinforcements have been used as onlays and inlays due to their high-longevity, comparable wear resistance to natural tooth enamel, natural translucency, radiopacity, and biocompatibility [14–18]. One major problem in the use of porcelains in dentistry is their potential for brittle, catastrophic fracture [12,19] that may be accentuated by machining damage [20,21]. As there is little published information available on the machining behavior of porcelains, one objective of the present study is to evaluate the machining responses of porcelains. The second objective is to characterize the machining behavior in a high-toughness zirconia ceramic recently
0043-1648/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0043-1648(03)00195-9
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developed [22] as an alternative to porcelains or glassceramics in posterior restorations. The surgical grade yttria-stabilized tetragonal zirconia polyscrystal (Y-TZP) with a high-fracture strength and toughness and an outstanding biocompatibility have been used in orthopedics for joint replacement for more than a decade [23,24] and in dentistry as implants, abutments, inlays, onlays, and crowns for several years [25]. The new colored zirconia ceramics are aesthetic and can be made to closely match the color of human teeth [22]. Y-TZP ceramics are fully dense with a fine microstructure consisting of submicrometer tetragonal grains. The tetragonal to monoclinic transformation is accompanied by volume dilatation that produces compressive stresses, making crack propagation more difficult and surface flaws less detrimental to fracture. Growth of microscopic flaws that form during processing or surface damage generated during service [12,19] is also impeded by the compressive stresses in the zirconia. Although the grinding behavior of zirconia with diamond wheels, and especially the effect of grinding on strength, have been studied (see for example [26]), the abrasive machining of zirconia with the dental handpiece and dental burs have not been investigated. The surface integrity after machining, achievable removal rates, and bur life are of primary interest when machining the tough zirconia. The purpose of the present study, therefore, is to investigate the abrasive machining characteristics of feldspathic porcelain and tetragonal zirconia with a dental handpiece and diamond burs that are commonly used in dentistry. The removal rates are measured for burs with three diamond grit sizes and the wear of burs is examined. The surface integrity, including surface roughness and edge chipping is evaluated as a function of diamond grit size. Microscopic examination of machined surfaces and machining debris is used to determine the mechanisms of material removal. The results of this study are compared with results of similar studies on other dental ceramics.
dine [27]. Reported [28] mechanical properties are: Vickers hardness = 6.3 ± 0.3 GPa, Young’s modulus = 69.7 GPa, and fracture toughness = 1.19 ± 0.05 MPa m1/2 . Specimens 3 mm × 4 mm × 25 mm in dimension were cut from blocks and all the surfaces were sand-blasted to achieve a uniform surface roughness. The yttria-stabilized tetragonal zirconia evaluated in this study was a commercial, surgical grade zirconia, Prozyr (Norton-St. Gobain, Northboro, MA), containing 3% mole fraction (5.1 ± 0.3% mass fraction) of yttria. It was prepared by isostatic pressing followed by sintering. The grain size distribution determined on a polished surface was relatively uniform with an average grain size of 0.6 m [27]. The reported [28] mechanical properties are: Vickers hardness = (13.9 ± 0.4) GPa, Young’s modulus = 210 GPa, fracture toughness = (4.9±0.2) MPa m1/2 . The test specimens were received in the form of bars with a rectangular cross-section (3 mm × 5 mm × 25 mm). The received specimens had been prepared by grinding with a 320-grit diamond wheel on all surfaces. 2.2. Test apparatus The machining apparatus utilized in this investigation has been described previously [6]. It incorporated a high-speed air-turbine-type handpiece equipped with an air–water mist delivery system (625C Super Torque, KaVo American Corp.). The dental handpiece was clamped to an arm that pivoted freely on a ball bearing support. The load between the bur and workpiece was established by a pulley and suspended weight arrangement. 2.3. Diamond burs Disposable dental diamond burs manufactured by NTI Diamond (Axis Dental Corporation, Irving, TX) were used. Each bur comprised a steel blank of fixed diameter for
2. Experimental procedure 2.1. Workpiece materials The dental porcelain selected for this study was Vita Mark II (Vita Zahnfabric, Bad Sackingen, Germany)2 commonly used for inlays, onlays and veneer restorations in the CAD/CAM Cerec system (Siemens, Bensheim, Germany). The microstructure analyzed in a scanning electron micrograph of a polished cross-section consisted of a glass matrix containing approximately 30% (volume fraction) of irregularly-shaped crystalline particles measuring from 1 to 7 m in size, some of which were crystalline sani2
Information on product names, manufacturers, and suppliers is included for clarification. This does not imply endorsement of the products or services by the National Institute of Standards and Technology.
Fig. 1. Schematic representation of groove cut into a test sample with a diamond bur, showing the designations used in the text for different surfaces: (1) top surface, (2) bottom surface, (3) inside surface, (4) side surface, and (5) front surface.
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mounting in the handpiece and a cutting portion that consisted of a single layer of synthetic diamond grit, electrolytically codeposited with nickel. Burs of three different grit sizes designated by the manufacturer as ultrafine (862 012UF), fine (862 012F), and coarse (862 012C) were used. The average diameter of the diamond particles were estimated from a series of micrographs obtained by scanning electron microscopy (SEM) on new burs to be 10±3, 41±17, and 172 ± 49 m, respectively. The burs had a pre-plated nominal diameter of 1.2 mm and a straight cylindrical geometry with a flame-shaped tip at the end. The burs were positioned so that only the straight cylindrical portion was in contact with the porcelain and zirconia workpieces (Fig. 1).
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The actual bur diameter in the diamond section was larger than the nominal diameter due to the added thickness of the plated layer and the diamond grit [6]. 2.4. Machining procedure The orientation of the bur during machining with respect to the specimen is illustrated in Fig. 1, which also shows schematically the various features associated with a single groove cut in the specimen. The bur was oriented approximately parallel to the 3 mm × 25 mm surface of the specimen (designated 5 in Fig. 1). A series of grooves was made along the length of each specimen.
Fig. 2. The volumetric removal rate as a function of total machining time for (a) porcelain and (b) zirconia cut with diamond burs of three different grit sizes (UF: ultrafine, F: fine, and C: course). The mean value from the repeat tests with three new burs of each grit size is represented by the data symbols. The uncertainty bars represent ± one standard deviation of the measured removal rates. The lines are empirical best fit to the data. Before the fourth cut at 40 s, the burs were ultrasonically cleaned for 30 s.
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Water coolant was delivered to the handpiece at a constant flow rate of (15 ± 2) ml/min during cutting. The handpiece was positioned such that the water spray nozzle was located at 180◦ with respect to the bur–specimen contact. Before each test series was begun, a light hydrocarbon lubricant (KaVo lubricant spray, KaVo American Corp.) was applied for approximately 1 s to lubricate the handpiece bearings. The handpiece was then run without load for 60 s, as suggested by Siegel and Von Fraunhofer [5], to remove the excess lubricant. A constant applied load of 2 N, typical of that used by clinicians in finishing operations [3,5], was used in the cutting experiments. At a constant air pressure of 241 ± 7 kPa (35 ± 1 psi), the pressure used throughout out this investigation, the rotational speed of the bur was approximately 320,000 rpm, measured by means of a strobe-type tachometer (Strobotac 1538-A, General Radio Company). On contacting the specimen under the 2 N load the speed dropped to 260,000 rpm where it remained until the handpiece was removed from the specimen. Three burs of each diamond grit size were used in the study for each test condition to allow for evaluation of the repeatability of the results. The machining sequence consisted of cutting a series of four grooves with each bur along the length of the porcelain and zirconia bars. The cutting duration for each groove was approximately 5 s for porcelain and 10 s for zirconia. A longer cutting duration was used for the zirconia to account for the lower removal rate. The burs were removed from the handpiece prior to the fourth cut and were ultrasonically cleaned in water for 30 s (Cole-Parmer Instrument Company, Chicago, IL). Bur cleaning was done to reduce the possible effect of bur loading on the removal rate.
(Rz ) were used to characterize surface roughness. Three traces were made at different positions along each groove. The mean values and the standard deviations of measurements were determined from the three repeat measurements on the three grooves cut with the three burs of each grit size. Several machined specimens were ultrasonically cleaned, sputter coated with gold, and examined in the SEM to assess the mechanisms of material removal during machining and to examine the groove edges for chipping damage. The quantity used to determine the degree of edge chipping at each groove was the projected chipped area per unit length (or average chipping width) measured along the groove edges
2.5. Characterization methods The volume removal rate for each groove was determined by dividing the groove volume by the time to cut the groove. The groove volume was determined by multiplying the average of the top and bottom groove areas by the specimen thickness. An image analysis system (Leica Microimage Video System with Image-Pro Plus) was used to measure the top and bottom groove areas. The actual cutting duration was determined from the cutting force data that were recorded as a function of time [6]. The means and standard deviations were determined from the measurements on the grooves that were cut with the three burs of each grit size for the same machining duration. Surface roughness was measured by means of a stylus profilometer (Perthen, Mahr GmbH). The traces perpendicular to the machining direction were made across the bottom of the grooves (designated 3 in Fig. 1) using a trace length of 1.75 mm and a cut off length of 0.25 mm. Arithmetic mean roughness (Ra ), maximum roughness (Rmax ), and mean roughness, i.e. the average of five largest distances between the highest and the lowest points within the sampling length
Fig. 3. SEM micrographs of the burs after machining porcelain: (a) ultrafine bur, (b) fine bur, and (c) coarse bur.
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on the top and bottom surfaces of the specimens (designated 1 and 2 in Fig. 1). The projected chipped area is the area of the chip projected on the plane of the top or bottom specimen surfaces. Measurements of the chipped area and the edge length of the grooves were made with the same image analysis system used to determine the groove volume. Following the cutting experiments, the burs were sputter coated with gold and examined by SEM, without prior cleaning, to determine the condition of the burs with respect to bur loading and grit wear and to identify the types of machining chips that were left on the burs. 2.6. Evaluation of bur wear A series of prolonged cutting tests was conducted to further evaluate the wear of diamond burs. In these tests, one bur of each grit size was used to cut 60 grooves in the porcelain (5 s duration each) for a total machining time of 300 s. Similarly, one fine bur was used to cut 72 grooves in the zirconia (10 s duration each) for a total machining time of 720 s. The burs were ultrasonically cleaned in water for 30 s several times during the tests. This procedure was followed to minimize the influence of bur loading by removing the machining debris attached to the bur. At the end of the tests, the material removal rate was measured for each groove; and the
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burs were ultrasonically cleaned, sputter coated with gold, and examined in the SEM to check for wear and grit pull out.
3. Results 3.1. Material removal rate The removal rates achieved with the diamond burs were higher for porcelain (Fig. 2(a)), than for zirconia (Fig. 2(b)), by at least one order of magnitude. The removal rate for both materials increased with an increase in the diamond grit size. The removal rate for porcelain with the coarse burs was approximately 15% higher than the removal rate with the fine burs (Fig. 2(a)), even though the grit size was more than three times larger. The removal rate for porcelain with the coarse burs was four to six times larger than the removal rate with the ultrafine burs. The removal rate for zirconia with the coarse burs was approximately 50% higher than the removal rate with the fine burs and 150% higher than the removal rate with the ultrafine burs (Fig. 2(b)). In general, the removal rates for both materials decreased as the total machining time was increased (Fig. 2). Ultrasonic cleaning of the burs prior to the fourth cut to remove bur
Fig. 4. SEM micrographs of zirconia machining debris on the burs: (a) small debris on an ultrafine bur, (b) a curled cutting chip on a fine bur, (c) debris on a coarse bur, and (d) elongated chips on a coarse bur.
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loading did not substantially affect the removal rate, with one exception. The removal rate for zirconia with the coarse burs stayed relatively constant for the first three cuts, but it decreased for the last cut, in spite of ultrasonic cleaning prior to making the cut. 3.2. Material removal mechanisms Examination in the SEM of the burs after machining did not reveal clear evidence of wear, despite a trend towards decreasing removal rate with increasing cutting time in these short tests. Examination of the burs used to cut the porcelain showed numerous, very fine, submicrometer de-
bris on the ultrafine and fine burs as well as a few larger particles (Fig. 3(a)), and agglomerated debris (Fig. 3(b)). No debris was seen on the coarse burs (Fig. 3(c)). The sharp edges on the larger particles and the fragmented nature of the fine debris suggest material removal by brittle fracture. Four types of machining debris were observed on the burs after cutting zirconia. The debris on the ultrafine burs consisted mostly of small acicular particles 1–2 m in length (Fig. 4(a)). Several ribbon-like curled chips, often associated with ductile cutting, as well as small fragmented particles were seen on the fine burs (Fig. 4(b)). Debris on the coarse burrs (Fig. 4(c)), consisted primarily of small equiaxed
Fig. 5. SEM micrographs of porcelain machined surfaces cut with ultrafine burs (a and d), fine burs (b and e), and coarse burs (c and f). The micrographs in (d–f) show the same surfaces as in (a–c) but at a higher magnification.
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particles with dimensions on the order 1 m or less, that are characteristic of chip formation by brittle fracture. Elongated chips, 20 m in length and 1–2 m in width, characteristic of segmented chips produced by ductile cutting, were also seen on the coarse burs (Fig. 4(d)). X-ray energy dispersive analysis in the SEM confirmed that the debris in Figs. 3 and 4 originated from the respective porcelain and zirconia workpieces. The machined porcelain and zirconia surfaces viewed in the SEM consisted mostly of a series of parallel scratches (Figs. 5 and 6). The width of these scratches that appeared to be formed by plastic deformation increased as the grit size was increased (see Fig. 5(a–c) for porcelain, and Fig. 6(a–c) for zirconia). The machined porcelain surfaces
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also contained features (pits with clearly defined edges) that were characteristic of a brittle removal process (i.e. brittle fracture). Examination of the machined surfaces at a higher magnification (Fig. 5(d–f)), indicated an increasing trend for microfracture as the grit size was increased. In contrast, the scratches on zirconia surfaces did not contain much evidence for brittle fracture and the smooth scratches were characteristic of a ductile removal process (i.e. plastic deformation). The zirconia surfaces machined with the ultrafine burs were mostly characterized with very smooth scratches associated with plowing or ductile cutting (Fig. 6(d)). The zirconia surfaces machined with the coarse burs contained characteristic features associated with plastic flow, delamination of deformed layer, and side flow across the scratches
Fig. 6. SEM micrographs of zirconia machined surfaces cut with ultrafine burs (a and d), fine burs (b and e), and coarse burs (c and f). The micrographs in (d–f) show the same surfaces as in (a–c) but at a higher magnification.
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(Fig. 6(f)). Similar features were seen on the zirconia surfaces machined with fine burs (Fig. 6(e)), but the occurrence of microfracture and delamination was less with the fine burs. 3.3. Surface integrity The measured surface roughness values, Ra , Rz , and Rmax , increased for both materials as the grit size was increased (Figs. 7 and 8). There was no effect of total machining time on the roughness parameters. The roughness appeared to be independent of the material being machined, and only the grit size had a major influence on roughness. The Ra roughness generated with the coarse burs was about three times as high as the Ra for the fine burs and 18 times as high as the Ra for the ultrafine burs. The increase in roughness was consistent with the SEM observations that showed wider
and deeper scratches as the grit size was increased (Figs. 5 and 6). Examination in the SEM of the machined grooves in the porcelain showed no chipping damage with the ultrafine burs (Fig. 9(a)), only slight chipping with the fine burs (Fig. 9(b)), and relatively large chipped areas along the edges when coarse burs were used (Fig. 9(c)). Examination of the machined grooves in zirconia showed no evidence for edge chipping (Fig. 10(a–c)), even for the grooves cut with the coarse burs (Fig. 10(c)). The average chipping width measured for porcelain (Fig. 11), confirmed the strong influence of diamond grit size on edge chipping in that material. The porcelain samples showed a 5–10-fold increase in the extent of chipping with the coarse burs compared with the fine burs. The standard deviations (Fig. 11), were much larger for the coarse burs than the fine burs, indicating the difficulty in maintaining a consistent edge quality with the coarse burs.
Fig. 7. Arithmetic average roughness (Ra ), 10 point height roughness (Rz ) and maximum roughness (Rmax ) for the grooves cut in porcelain with coarse, fine, and ultrafine diamond burs. Each data point is the mean value from three repeat cuts with three different burs under the same machining condition; the error bars represent ± one standard deviation for the cuts.
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Fig. 8. Arithmetic average roughness (Ra ), 10 point height roughness (Rz ) and maximum roughness (Rmax ) for the grooves cut in zirconia with coarse, fine, and ultrafine diamond burs. Each data point is the mean value from three repeat cuts with three different burs under the same machining conditions; the error bars represent ± one standard deviation for the cuts.
The average chipping widths with the ultrafine burs were negligible. 3.4. Wear of diamond burs The removal rate of porcelain after prolonged tests (300 s of machining) was reduced by 70, 90 and 80% for the ultrafine, fine and coarse burs (Fig. 12(a)), presumably due to wear of the burs. The removal rate of zirconia after 720 s of machining with the fine bur was reduced by only 50% (Fig. 12(b)). Examination of the burs in the SEM after the prolonged cutting tests and following ultrasonic cleaning to remove the machining debris revealed four types of wear damage: grit dislodgment, grit fracture, attritious wear, and matrix abrasion. The extent of each damage type was different for each workpiece and the diamond grit size. Grit
dislodgment or pullout (Fig. 13(a)), probably due to poorly held particles in the matrix or application of excessive grit load, accounted for about 20% diamond loss in the ultrafine bur when machining porcelain. Although grit dislodgment accounted for about 8 and 9% diamond loss after machining porcelain with the fine and coarse burs, grit fracture, where large portions of the grit were cleaved off (Fig. 13(b–c)), was found to be the dominant wear process in these burs. Similarly, while grit dislodgment accounted for about 15% diamond loss (Fig. 14(a)), when machining zirconia with the fine burs, grit fracture (Fig. 14(b)), was found to be the dominant wear process. Several diamond grit were found to contain wear flats, where the grit edges had been worn producing small flat surfaces (Fig. 14(c)). Wear flat generation, or attritious wear, was also observed in burs used for porcelain machining, but it was not a common wear process with
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Fig. 9. SEM micrographs showing typical chipping damage around the groove top surfaces for porcelain cut with (a) an ultrafine bur, (b) a fine bur, and (c) a coarse bur.
either material. Abrasion damage to the nickel matrix was also observed (Fig. 14(b)).
4. Discussion In this study, we have compared the abrasive machining behavior of dental porcelain with a high-strength zirconia using dental burs of three different grit sizes. The results clearly show a distinct difference in response for the two materials, both with respect to the removal rates achieved during cutting and the basic removal mechanisms. The removal rates for the porcelain were larger by at least one order of magnitude than for zirconia. While the removal
Fig. 10. SEM micrographs showing no chipping damage around the groove top surfaces for zirconia cut with (a) an ultrafine bur, (b) a fine bur, and (c) a coarse bur.
rates for the porcelain were equivalent to that of Dicor, a machinable glass ceramic [6], the removal rate of zirconia was close to the rate obtained for InCeram alumina [7]. These four ceramics seem to show a correlation between the removal rate and hardness. However, a more systematic analysis is needed before an authoritative statement can be made. Although the removal rates of porcelain and zirconia are significantly different, the surface roughness parameters are almost indistinguishable when the same bur grit size is used. This material-independent roughness behavior is consistent with the previous results for glass-ceramics [6] and glass-infiltrated alumina [7], and with a Monte Carlo simulation of surface roughness generated by abrasive machining
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Fig. 11. Average chipping width around the groove surfaces in porcelain cut with coarse, fine, and ultrafine diamond burs. Each data point is the mean value from three repeat cuts with three different burs under the same machining conditions; the error bars represent ± one standard deviation for the cuts.
[29]. In that study, surface roughness was shown to be primarily sensitive to the grit size, consistent with the present results. Diamond grit size plays an important role in abrasive machining. It is generally believed that higher removal rates can be achieved by coarser grit burs, and therefore, coarse burs are often used for gross tooth reduction [8]. The relationship between the removal rate and grit size, however, is not necessarily linear for all materials. While the removal rate for porcelain was higher by about 15% with coarse burs than with fine burs, the removal rate for zirconia increased by nearly 50% upon the use of coarse burs. The removal rate for machinable glass-ceramics did not always increase with the use of coarse burs [6]. However, coarse burs substantially increased the extent of chipping damage along the edges of the grooves cut in glass-ceramics [6], similar to the results on porcelain in the present study. The zirconia ceramic, unlike the porcelain, did not exhibit any visible edge chipping with any of the burs used in this study. Since edge retention at the margins and the achievement of close dimensional tolerances are important issues in obtaining high-quality restorations, one must balance the high-removal rates often associated with the use of coarse burs with the potential loss of surface/edge integrity. The use of coarse burs could also increase the propensity for generation of subsurface cracks with concomitant strength degradation [20]. The strength reduction due to machining-induced damage is, however, material-dependent. The yttria-stabilized tetragonal zirconia is less sensitive to machining damage compared to many other polycrystalline ceramics [26]. The effect of grit size on removal rate and edge chipping is not necessarily a simple phenomenon. Usually, an
increase in the grit size is accompanied by a reduction in the number of diamond particles in dental burs [6]. Therefore, with the same load is applied to the bur, the grit load can be much larger with a coarser bur. The higher grit load causes an increased grit penetration into the ceramic workpiece resulting in a higher removal rate and generation of surface and subsurface cracks. Both the removal rate and the extent of chipping damage depend on the same properties that control deformation and fracture of the ceramics, i.e. hardness and toughness [30], that also control the mode of material removal, i.e. brittle fracture versus plastic deformation [20]. The material removal mechanism in abrasive machining of porcelain was dominated by brittle fracture, as seen by the pits with sharp edges on the machined surfaces (Fig. 5) and the fragmented appearance of the machining debris (Fig. 3). However, the removal mechanisms for zirconia were primarily plastic deformation and microcutting, as evidenced by the smooth furrows on the machined surfaces (Fig. 6) and the ribbon-like shape of the machining debris (Fig. 4). The difference in the behavior of these two materials with respect to the predominance of brittle fracture or plastic deformation is consistent with Hertzian contact studies [27]. For the porcelain, the response to indentation with a tungsten carbide sphere was classical brittle fracture, i.e. formation of a well defined cone crack [27]. The response for zirconia, however, was primarily plastic or quasi-plastic, i.e. evolution of a diffuse subsurface zone of microscopic shear cracks [27]. The brittle/ductile response of these two materials to indentation can also be predicted by using a brittleness index proposed by Rhee et al. [31], derived from the ratio of the contact load needed for initiation of subsurface plastic
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Fig. 12. The volumetric removal rate as a function of total machining time in prolonged cutting tests for (a) porcelain and (b) zirconia. Diamond burs of three different grit sizes (ultrafine, fine, and course) were used in the tests with porcelain, while only a fine bur was used with zirconia. The burs were ultrasonically cleaned several times during the test series.
yield to the threshold load for the formation of ring cracks. The distinction between the brittle/ductile behavior of these two materials can also be predicted by using a similar brittleness index proposed by Quinn and Quinn [32], related to the ratio of deformation energy to fracture energy. The disposable or single-patient diamond burs employed in the present study are chosen by the clinicians to minimize cross-contamination risks of bloodborne pathogens [33]. It is likely that such burs would be used no more than a few tens of seconds before being discarded. However, our results indicate that these single-patient burs can be used for several minutes without a significant loss of cutting efficiency.
The cutting efficiency is measured by the reduction in the removal rate as a function of cutting time. The decrease in the removal rate as a function of cutting time is generally due to bur loading (attachment of machining debris to the bur) and wear of the diamond particles. The influence of bur loading in our study, however, was minimal since ultrasonic cleaning of the burs prior to the fourth cuts did not significantly increase the removal rate (Fig. 2). Wear of the burs occurred through grit fracture, grit pullout, wear flat generation or attritious wear, and matrix damage. These results are consistent with the observed wear mechanisms previously discussed for dental cutting
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Fig. 13. SEM micrographs of the burs after prolonged cutting tests on porcelain: (a) grit dislodgment on the ultrafine bur, (b) grit fracture on the fine bur, and (c) grit fracture on the coarse bur.
of glass-infiltrated alumina [7]. The observed grit fracture suggests that a friable (self-sharpening) diamond had been used in the burs, which may explain the fairly high-cutting efficiency of the burs in prolonged tests. The wear mechanisms in diamond burs can also be influenced by the grit size. It is generally accepted that finer diamond particles are stronger than the larger ones due to the lower flaw population as the original flaws in the large particles are eliminated by fracture when finer grit is made by crushing the coarse diamond particles [34]. Therefore, less grit fracture is expected for finer burs. However, fine burs are more prone to matrix damage since the space available for debris removal is smaller for the fine burs. The machin-
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Fig. 14. SEM micrographs of a fine bur prolonged cutting tests on zirconia showing different types of wear and damage: (a) grit dislodgment, (b) grit fracture and matrix abrasion, and (c) wear flat generation.
ing debris could cause abrasion damage on the metal matrix used to hold the diamond particles on the bur, weakening the bond between the diamond particles and the matrix, and resulting in grit loss. Therefore, grit loss becomes a more dominant wear process for finer burs.
5. Conclusions In this study, we investigated the machining characteristics of feldspathic porcelain and tetragonal zirconia with
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a dental handpiece and diamond burs of varying grit sizes under simulated clinical cutting conditions. The following conclusions are drawn from the results: (1) As the diamond grit size was increased from ultrafine (10 m) to fine (40 m) and coarse (151 m) the removal rate and the resulting surface roughness for both materials increased substantially. (2) The removal rate for porcelain was much higher than for zirconia for all grit sizes. (3) The porcelain sustained severe edge chipping with both coarse and fine burs, but edge chipping was essentially absent with the ultrafine burs. The zirconia was free of edge chipping for all grit sizes. (4) The surface roughness increased with increasing grit size for both porcelain and zirconia, and it was materialindependent. (5) The mechanisms of material removal determined through microscopic examination of the machined surfaces and the machining debris on the burs were found to consist of a combination of ductile and brittle-type chip formation processes. The occurrence of brittle fracture increased as the diamond grit size was increased. While the material removal process in the porcelain was dominated by brittle fracture, the removal process in zirconia was dominated by ductile cutting. (6) Four wear processes were identified on the burs in prolonged cutting tests: grit microfracture, grit pullout, wear flat generation, and matrix abrasion.
Acknowledgements We acknowledge the generous supply of materials from Norton St. Gobain and Vita Zhanfabrik. This project was supported by NIDCR Program Project Grant No. P01 DE 10976. We are grateful to Professors E.D. Rekow and V.P. Thompson of the New York University College of Dentistry for their overall guidance of this program.
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[5] S.C. Siegel, J.A. Von Fraunhofer, Effect of handpiece load on the cutting efficiency of dental burs, Mach. Sci. Technol. 1 (1) (1997) 1–13. [6] X. Dong, L. Yin, S. Jahanmir, L.K. Ives, E.D. Rekow, Abrasive machining of glass-ceramics with a dental handpiece, Mach. Sci. Technol. 4 (2) (2000) 209–233. [7] L. Yin, L.K. Ives, S. Jahanmir, E.D. Rekow, E. Romberg, Machining characteristics of glass-infiltrated alumina with diamond burs, Mach. Sci. Technol. 5 (1) (2001) 43–61. [8] S.C. Siegel, J.A. Von Fraunhofer, Dental cutting: the historical development of diamond burs, J. Am. Dent. Assoc. 129 (6) (1998) 740–745. [9] E.D. Rekow, A.G. Erdman, D.R. Riley, B. Klamecki, CAD/CAM for dental restorations—some of the curious challenges, IEEE Trans. Biomed. Eng. 38 (4) (1991) 314–318. [10] E.D. Rekow, Dental CAD/CAM systems, J. Am. Dent. Assoc. 122 (13) (1991) 42–48. [11] M. Friedman, Augmenting restorative dentistry with porcelain veneers, J. Am. Dent. Assoc. 122 (6) (1991) 29–34. [12] J.R. Kelly, Ceramics in restorative and prosthetic dentistry, Mater. Sci. 27 (1997) 443–468. [13] M. Rosenblum, A. Schulman, A review of all-ceramic restorations, J. Am. Dent. Assoc. 128 (3) (1997) 297–307. [14] I. Denry, J. Mackert, L. Holloway, S. Rosentiel, Effect of cubic leucite stabilization on the flexural strength of feldspathic dental porcelain, J. Dent. Res. 75 (12) (1996) 1928–1935. [15] R. Seghi, I. Dnery, S. Rosenstiel, Relative fracture toughness and hardness of new dental ceramics, J. Prosthet. Dent. 74 (2) (1995) 145–150. [16] I. Krejci, F. Lutz, M. Reimer, Wear of CAD/CAM ceramic inlays: restorations, opposing cups, and luting cements, Quintessence Int. 25 (3) (1994) 199–207. [17] H. Schwickerath, The strength characteristics of cerec, Quintessenz 43 (1992) 669–677. [18] I. Krejci, in: W. Mormann (Ed.), Wear of cerec and other restorative materials, Proceedings of the International Symposium on Computer Restorations, Quintessence Publishing, Chicago, IL, USA, 1991, pp. 245–251. [19] J.R. Kelly, I. Nishimura, S.D. Campbell, Ceramics in dentistry: historical roots and current perspectives, J. Prosthet. Dent. 75 (1) (1996) 18–32. [20] S. Jahanmir, H.H.K. Xu, L.K. Ives, in: S. Jahanmir, P. Koshy, M. Ramulu (Eds.), Mechanism of Materials Removal in Abrasive Machining of Ceramics, Machining of Ceramics and Composites, Marcel Dekker, New York, 1999, pp. 11–84. [21] H.H.K. Xu, S. Jahanmir, L.K. Ives, Material removal and damage formation mechanisms in grinding silicon nitride, J. Mater. Res. 11 (7) (1996) 1717–1724. [22] B. Cales, Colored zirconia ceramics for dental applications, Bioceramics 11 (1998) 591–594. [23] B. Cales, Zirconia ceramic for improved hip prosthesis—a review, in: Proceedings of the Sixth Biomaterials Symposium on Ceramic Implant Materials in Orthopedic Surgery, Gottingen, Germany, 1994. [24] J. Chevalier, J.M. Drouin, B. Cales, in: L. Sedel, C. Rey (Eds.), Low Temperature Aging Behavior of Zirconia Hip Joint Heads, Proceedings of the Tenth International Symposium on Ceramics in Medicine, Elsevier, Amsterdam, 1997. [25] Active Biomedicale, Norton Desmarquest, 1998. [26] H.H.K. Xu, S. Jahanmir, L.K. Ives, Effect of grinding on strength of tetragonal zirconia and zirconia-toughened alumina, Mach. Sci. Technol. 1 (1) (1997) 49–66. [27] I.M. Peterson, A. Pajares, B.R. Lawn, V.P. Thomspon, E.D. Rekow, Mechanical characterization of dental ceramics by hertzian contacts, J. Dent. Res. 77 (4) (1998) 589–602. [28] J. Quinn, L. Su, L. Flanders, L. Lloyd, Edge toughness and material properties related to the machining of dental ceramics, Mach. Sci. Technol. 4 (2) (2000) 291–304.
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Abrasive machining of porcelain and zirconia with a dental handpiece L. Yin1 , S. Jahanmir∗ , L.K. Ives National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899-8521, USA
Abstract The machining characteristics and material removal mechanisms of two dental ceramics—feldspathic porcelain and yttria-stabilized tetragonal zirconia—were investigated using a dental handpiece and diamond burs with different grit sizes. The material removal rates were measured as a function of total machining time using a constant load of 2 N on the bur, consistent with clinical cutting conditions. As the diamond grit size was increased from ultrafine (UF) (10 m) to fine (F) (41 m) and coarse (C) (172 m), the removal rate and the resulting surface roughness for each material increased substantially. The mechanisms of material removal determined through microscopic examination of the machined surfaces and the machining debris on the burs were found to consist of a combination of ductile and brittle-type chip formation processes. The occurrence of brittle fracture increased as the diamond grit size was increased. While the material removal process in porcelain was dominated by brittle fracture, zirconia was primarily subjected to ductile cutting. Four wear processes were identified on the burs in prolonged cutting tests: grit microfracture, grit pullout, wear flat generation, and matrix abrasion. The results demonstrated that while the material removal rate for the zirconia evaluated in this study was lower than those for porcelain and many other dental ceramics, the zirconia could be machined under clinical conditions with no edge chipping damage. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Dental restorations; Diamond burs; Dental handpiece; Yttria-stabilized tetragonal zirconia; Felspathic porcelain; Material removal mechanism; Tool wear; Surface integrity
1. Introduction Abrasive machining is routinely used in dentistry for the preparation of teeth for filling, crowns, and bridgework, and for making final fit adjustments on dental restorations [1]. In these machining procedures, a high-speed, air-driven dental handpiece with a diamond bur is often used. In spite of the importance of diamond machining in dentistry, there are only a few published studies on the subject [2–8]. Recent studies have provided data on the machining characteristics of tooth enamel [4], machinable glass-ceramics [6], and glass-infiltrated alumina [7] using a dental handpiece and diamond burs. The purpose of the present study is to investigate the abrasive machining characteristics of a commonly used dental porcelain and a new tetragonal zirconia. In addition to handpiece machining performed by the dentist, abrasive machining is used to prepare ceramic restorations in the dental laboratory using computer-aided ∗ Corresponding author. Present address: MiTiHeart Corporation, P.O. Box 83610, Gaithersburg, MD 20883, USA. Fax: +1-301-869-9724. E-mail address: [email protected] (S. Jahanmir). 1 Present address: Gintic Institute of Manufacturing Technology, Singapore 638075, Singapore.
design and computer-aided manufacturing (CAD/CAM) systems [9,10]. The desired shapes for crowns, onlays, or inlays are obtained fairly rapidly using either a milling cutter or an abrasive tool depending on the properties of the workpiece material. The data obtained in abrasive machining of porcelain and zirconia with the dental handpiece can also be used as a guide for CAD/CAM preparation of dental restorations with these ceramics, where similar machining conditions apply. Porcelains have been widely used for dental restorations as veneers mainly because of their excellent esthetics [11–13]. More recently, modified feldspathic porcelains containing crystalline reinforcements have been used as onlays and inlays due to their high-longevity, comparable wear resistance to natural tooth enamel, natural translucency, radiopacity, and biocompatibility [14–18]. One major problem in the use of porcelains in dentistry is their potential for brittle, catastrophic fracture [12,19] that may be accentuated by machining damage [20,21]. As there is little published information available on the machining behavior of porcelains, one objective of the present study is to evaluate the machining responses of porcelains. The second objective is to characterize the machining behavior in a high-toughness zirconia ceramic recently
0043-1648/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0043-1648(03)00195-9
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developed [22] as an alternative to porcelains or glassceramics in posterior restorations. The surgical grade yttria-stabilized tetragonal zirconia polyscrystal (Y-TZP) with a high-fracture strength and toughness and an outstanding biocompatibility have been used in orthopedics for joint replacement for more than a decade [23,24] and in dentistry as implants, abutments, inlays, onlays, and crowns for several years [25]. The new colored zirconia ceramics are aesthetic and can be made to closely match the color of human teeth [22]. Y-TZP ceramics are fully dense with a fine microstructure consisting of submicrometer tetragonal grains. The tetragonal to monoclinic transformation is accompanied by volume dilatation that produces compressive stresses, making crack propagation more difficult and surface flaws less detrimental to fracture. Growth of microscopic flaws that form during processing or surface damage generated during service [12,19] is also impeded by the compressive stresses in the zirconia. Although the grinding behavior of zirconia with diamond wheels, and especially the effect of grinding on strength, have been studied (see for example [26]), the abrasive machining of zirconia with the dental handpiece and dental burs have not been investigated. The surface integrity after machining, achievable removal rates, and bur life are of primary interest when machining the tough zirconia. The purpose of the present study, therefore, is to investigate the abrasive machining characteristics of feldspathic porcelain and tetragonal zirconia with a dental handpiece and diamond burs that are commonly used in dentistry. The removal rates are measured for burs with three diamond grit sizes and the wear of burs is examined. The surface integrity, including surface roughness and edge chipping is evaluated as a function of diamond grit size. Microscopic examination of machined surfaces and machining debris is used to determine the mechanisms of material removal. The results of this study are compared with results of similar studies on other dental ceramics.
dine [27]. Reported [28] mechanical properties are: Vickers hardness = 6.3 ± 0.3 GPa, Young’s modulus = 69.7 GPa, and fracture toughness = 1.19 ± 0.05 MPa m1/2 . Specimens 3 mm × 4 mm × 25 mm in dimension were cut from blocks and all the surfaces were sand-blasted to achieve a uniform surface roughness. The yttria-stabilized tetragonal zirconia evaluated in this study was a commercial, surgical grade zirconia, Prozyr (Norton-St. Gobain, Northboro, MA), containing 3% mole fraction (5.1 ± 0.3% mass fraction) of yttria. It was prepared by isostatic pressing followed by sintering. The grain size distribution determined on a polished surface was relatively uniform with an average grain size of 0.6 m [27]. The reported [28] mechanical properties are: Vickers hardness = (13.9 ± 0.4) GPa, Young’s modulus = 210 GPa, fracture toughness = (4.9±0.2) MPa m1/2 . The test specimens were received in the form of bars with a rectangular cross-section (3 mm × 5 mm × 25 mm). The received specimens had been prepared by grinding with a 320-grit diamond wheel on all surfaces. 2.2. Test apparatus The machining apparatus utilized in this investigation has been described previously [6]. It incorporated a high-speed air-turbine-type handpiece equipped with an air–water mist delivery system (625C Super Torque, KaVo American Corp.). The dental handpiece was clamped to an arm that pivoted freely on a ball bearing support. The load between the bur and workpiece was established by a pulley and suspended weight arrangement. 2.3. Diamond burs Disposable dental diamond burs manufactured by NTI Diamond (Axis Dental Corporation, Irving, TX) were used. Each bur comprised a steel blank of fixed diameter for
2. Experimental procedure 2.1. Workpiece materials The dental porcelain selected for this study was Vita Mark II (Vita Zahnfabric, Bad Sackingen, Germany)2 commonly used for inlays, onlays and veneer restorations in the CAD/CAM Cerec system (Siemens, Bensheim, Germany). The microstructure analyzed in a scanning electron micrograph of a polished cross-section consisted of a glass matrix containing approximately 30% (volume fraction) of irregularly-shaped crystalline particles measuring from 1 to 7 m in size, some of which were crystalline sani2
Information on product names, manufacturers, and suppliers is included for clarification. This does not imply endorsement of the products or services by the National Institute of Standards and Technology.
Fig. 1. Schematic representation of groove cut into a test sample with a diamond bur, showing the designations used in the text for different surfaces: (1) top surface, (2) bottom surface, (3) inside surface, (4) side surface, and (5) front surface.
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mounting in the handpiece and a cutting portion that consisted of a single layer of synthetic diamond grit, electrolytically codeposited with nickel. Burs of three different grit sizes designated by the manufacturer as ultrafine (862 012UF), fine (862 012F), and coarse (862 012C) were used. The average diameter of the diamond particles were estimated from a series of micrographs obtained by scanning electron microscopy (SEM) on new burs to be 10±3, 41±17, and 172 ± 49 m, respectively. The burs had a pre-plated nominal diameter of 1.2 mm and a straight cylindrical geometry with a flame-shaped tip at the end. The burs were positioned so that only the straight cylindrical portion was in contact with the porcelain and zirconia workpieces (Fig. 1).
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The actual bur diameter in the diamond section was larger than the nominal diameter due to the added thickness of the plated layer and the diamond grit [6]. 2.4. Machining procedure The orientation of the bur during machining with respect to the specimen is illustrated in Fig. 1, which also shows schematically the various features associated with a single groove cut in the specimen. The bur was oriented approximately parallel to the 3 mm × 25 mm surface of the specimen (designated 5 in Fig. 1). A series of grooves was made along the length of each specimen.
Fig. 2. The volumetric removal rate as a function of total machining time for (a) porcelain and (b) zirconia cut with diamond burs of three different grit sizes (UF: ultrafine, F: fine, and C: course). The mean value from the repeat tests with three new burs of each grit size is represented by the data symbols. The uncertainty bars represent ± one standard deviation of the measured removal rates. The lines are empirical best fit to the data. Before the fourth cut at 40 s, the burs were ultrasonically cleaned for 30 s.
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Water coolant was delivered to the handpiece at a constant flow rate of (15 ± 2) ml/min during cutting. The handpiece was positioned such that the water spray nozzle was located at 180◦ with respect to the bur–specimen contact. Before each test series was begun, a light hydrocarbon lubricant (KaVo lubricant spray, KaVo American Corp.) was applied for approximately 1 s to lubricate the handpiece bearings. The handpiece was then run without load for 60 s, as suggested by Siegel and Von Fraunhofer [5], to remove the excess lubricant. A constant applied load of 2 N, typical of that used by clinicians in finishing operations [3,5], was used in the cutting experiments. At a constant air pressure of 241 ± 7 kPa (35 ± 1 psi), the pressure used throughout out this investigation, the rotational speed of the bur was approximately 320,000 rpm, measured by means of a strobe-type tachometer (Strobotac 1538-A, General Radio Company). On contacting the specimen under the 2 N load the speed dropped to 260,000 rpm where it remained until the handpiece was removed from the specimen. Three burs of each diamond grit size were used in the study for each test condition to allow for evaluation of the repeatability of the results. The machining sequence consisted of cutting a series of four grooves with each bur along the length of the porcelain and zirconia bars. The cutting duration for each groove was approximately 5 s for porcelain and 10 s for zirconia. A longer cutting duration was used for the zirconia to account for the lower removal rate. The burs were removed from the handpiece prior to the fourth cut and were ultrasonically cleaned in water for 30 s (Cole-Parmer Instrument Company, Chicago, IL). Bur cleaning was done to reduce the possible effect of bur loading on the removal rate.
(Rz ) were used to characterize surface roughness. Three traces were made at different positions along each groove. The mean values and the standard deviations of measurements were determined from the three repeat measurements on the three grooves cut with the three burs of each grit size. Several machined specimens were ultrasonically cleaned, sputter coated with gold, and examined in the SEM to assess the mechanisms of material removal during machining and to examine the groove edges for chipping damage. The quantity used to determine the degree of edge chipping at each groove was the projected chipped area per unit length (or average chipping width) measured along the groove edges
2.5. Characterization methods The volume removal rate for each groove was determined by dividing the groove volume by the time to cut the groove. The groove volume was determined by multiplying the average of the top and bottom groove areas by the specimen thickness. An image analysis system (Leica Microimage Video System with Image-Pro Plus) was used to measure the top and bottom groove areas. The actual cutting duration was determined from the cutting force data that were recorded as a function of time [6]. The means and standard deviations were determined from the measurements on the grooves that were cut with the three burs of each grit size for the same machining duration. Surface roughness was measured by means of a stylus profilometer (Perthen, Mahr GmbH). The traces perpendicular to the machining direction were made across the bottom of the grooves (designated 3 in Fig. 1) using a trace length of 1.75 mm and a cut off length of 0.25 mm. Arithmetic mean roughness (Ra ), maximum roughness (Rmax ), and mean roughness, i.e. the average of five largest distances between the highest and the lowest points within the sampling length
Fig. 3. SEM micrographs of the burs after machining porcelain: (a) ultrafine bur, (b) fine bur, and (c) coarse bur.
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on the top and bottom surfaces of the specimens (designated 1 and 2 in Fig. 1). The projected chipped area is the area of the chip projected on the plane of the top or bottom specimen surfaces. Measurements of the chipped area and the edge length of the grooves were made with the same image analysis system used to determine the groove volume. Following the cutting experiments, the burs were sputter coated with gold and examined by SEM, without prior cleaning, to determine the condition of the burs with respect to bur loading and grit wear and to identify the types of machining chips that were left on the burs. 2.6. Evaluation of bur wear A series of prolonged cutting tests was conducted to further evaluate the wear of diamond burs. In these tests, one bur of each grit size was used to cut 60 grooves in the porcelain (5 s duration each) for a total machining time of 300 s. Similarly, one fine bur was used to cut 72 grooves in the zirconia (10 s duration each) for a total machining time of 720 s. The burs were ultrasonically cleaned in water for 30 s several times during the tests. This procedure was followed to minimize the influence of bur loading by removing the machining debris attached to the bur. At the end of the tests, the material removal rate was measured for each groove; and the
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burs were ultrasonically cleaned, sputter coated with gold, and examined in the SEM to check for wear and grit pull out.
3. Results 3.1. Material removal rate The removal rates achieved with the diamond burs were higher for porcelain (Fig. 2(a)), than for zirconia (Fig. 2(b)), by at least one order of magnitude. The removal rate for both materials increased with an increase in the diamond grit size. The removal rate for porcelain with the coarse burs was approximately 15% higher than the removal rate with the fine burs (Fig. 2(a)), even though the grit size was more than three times larger. The removal rate for porcelain with the coarse burs was four to six times larger than the removal rate with the ultrafine burs. The removal rate for zirconia with the coarse burs was approximately 50% higher than the removal rate with the fine burs and 150% higher than the removal rate with the ultrafine burs (Fig. 2(b)). In general, the removal rates for both materials decreased as the total machining time was increased (Fig. 2). Ultrasonic cleaning of the burs prior to the fourth cut to remove bur
Fig. 4. SEM micrographs of zirconia machining debris on the burs: (a) small debris on an ultrafine bur, (b) a curled cutting chip on a fine bur, (c) debris on a coarse bur, and (d) elongated chips on a coarse bur.
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loading did not substantially affect the removal rate, with one exception. The removal rate for zirconia with the coarse burs stayed relatively constant for the first three cuts, but it decreased for the last cut, in spite of ultrasonic cleaning prior to making the cut. 3.2. Material removal mechanisms Examination in the SEM of the burs after machining did not reveal clear evidence of wear, despite a trend towards decreasing removal rate with increasing cutting time in these short tests. Examination of the burs used to cut the porcelain showed numerous, very fine, submicrometer de-
bris on the ultrafine and fine burs as well as a few larger particles (Fig. 3(a)), and agglomerated debris (Fig. 3(b)). No debris was seen on the coarse burs (Fig. 3(c)). The sharp edges on the larger particles and the fragmented nature of the fine debris suggest material removal by brittle fracture. Four types of machining debris were observed on the burs after cutting zirconia. The debris on the ultrafine burs consisted mostly of small acicular particles 1–2 m in length (Fig. 4(a)). Several ribbon-like curled chips, often associated with ductile cutting, as well as small fragmented particles were seen on the fine burs (Fig. 4(b)). Debris on the coarse burrs (Fig. 4(c)), consisted primarily of small equiaxed
Fig. 5. SEM micrographs of porcelain machined surfaces cut with ultrafine burs (a and d), fine burs (b and e), and coarse burs (c and f). The micrographs in (d–f) show the same surfaces as in (a–c) but at a higher magnification.
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particles with dimensions on the order 1 m or less, that are characteristic of chip formation by brittle fracture. Elongated chips, 20 m in length and 1–2 m in width, characteristic of segmented chips produced by ductile cutting, were also seen on the coarse burs (Fig. 4(d)). X-ray energy dispersive analysis in the SEM confirmed that the debris in Figs. 3 and 4 originated from the respective porcelain and zirconia workpieces. The machined porcelain and zirconia surfaces viewed in the SEM consisted mostly of a series of parallel scratches (Figs. 5 and 6). The width of these scratches that appeared to be formed by plastic deformation increased as the grit size was increased (see Fig. 5(a–c) for porcelain, and Fig. 6(a–c) for zirconia). The machined porcelain surfaces
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also contained features (pits with clearly defined edges) that were characteristic of a brittle removal process (i.e. brittle fracture). Examination of the machined surfaces at a higher magnification (Fig. 5(d–f)), indicated an increasing trend for microfracture as the grit size was increased. In contrast, the scratches on zirconia surfaces did not contain much evidence for brittle fracture and the smooth scratches were characteristic of a ductile removal process (i.e. plastic deformation). The zirconia surfaces machined with the ultrafine burs were mostly characterized with very smooth scratches associated with plowing or ductile cutting (Fig. 6(d)). The zirconia surfaces machined with the coarse burs contained characteristic features associated with plastic flow, delamination of deformed layer, and side flow across the scratches
Fig. 6. SEM micrographs of zirconia machined surfaces cut with ultrafine burs (a and d), fine burs (b and e), and coarse burs (c and f). The micrographs in (d–f) show the same surfaces as in (a–c) but at a higher magnification.
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(Fig. 6(f)). Similar features were seen on the zirconia surfaces machined with fine burs (Fig. 6(e)), but the occurrence of microfracture and delamination was less with the fine burs. 3.3. Surface integrity The measured surface roughness values, Ra , Rz , and Rmax , increased for both materials as the grit size was increased (Figs. 7 and 8). There was no effect of total machining time on the roughness parameters. The roughness appeared to be independent of the material being machined, and only the grit size had a major influence on roughness. The Ra roughness generated with the coarse burs was about three times as high as the Ra for the fine burs and 18 times as high as the Ra for the ultrafine burs. The increase in roughness was consistent with the SEM observations that showed wider
and deeper scratches as the grit size was increased (Figs. 5 and 6). Examination in the SEM of the machined grooves in the porcelain showed no chipping damage with the ultrafine burs (Fig. 9(a)), only slight chipping with the fine burs (Fig. 9(b)), and relatively large chipped areas along the edges when coarse burs were used (Fig. 9(c)). Examination of the machined grooves in zirconia showed no evidence for edge chipping (Fig. 10(a–c)), even for the grooves cut with the coarse burs (Fig. 10(c)). The average chipping width measured for porcelain (Fig. 11), confirmed the strong influence of diamond grit size on edge chipping in that material. The porcelain samples showed a 5–10-fold increase in the extent of chipping with the coarse burs compared with the fine burs. The standard deviations (Fig. 11), were much larger for the coarse burs than the fine burs, indicating the difficulty in maintaining a consistent edge quality with the coarse burs.
Fig. 7. Arithmetic average roughness (Ra ), 10 point height roughness (Rz ) and maximum roughness (Rmax ) for the grooves cut in porcelain with coarse, fine, and ultrafine diamond burs. Each data point is the mean value from three repeat cuts with three different burs under the same machining condition; the error bars represent ± one standard deviation for the cuts.
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Fig. 8. Arithmetic average roughness (Ra ), 10 point height roughness (Rz ) and maximum roughness (Rmax ) for the grooves cut in zirconia with coarse, fine, and ultrafine diamond burs. Each data point is the mean value from three repeat cuts with three different burs under the same machining conditions; the error bars represent ± one standard deviation for the cuts.
The average chipping widths with the ultrafine burs were negligible. 3.4. Wear of diamond burs The removal rate of porcelain after prolonged tests (300 s of machining) was reduced by 70, 90 and 80% for the ultrafine, fine and coarse burs (Fig. 12(a)), presumably due to wear of the burs. The removal rate of zirconia after 720 s of machining with the fine bur was reduced by only 50% (Fig. 12(b)). Examination of the burs in the SEM after the prolonged cutting tests and following ultrasonic cleaning to remove the machining debris revealed four types of wear damage: grit dislodgment, grit fracture, attritious wear, and matrix abrasion. The extent of each damage type was different for each workpiece and the diamond grit size. Grit
dislodgment or pullout (Fig. 13(a)), probably due to poorly held particles in the matrix or application of excessive grit load, accounted for about 20% diamond loss in the ultrafine bur when machining porcelain. Although grit dislodgment accounted for about 8 and 9% diamond loss after machining porcelain with the fine and coarse burs, grit fracture, where large portions of the grit were cleaved off (Fig. 13(b–c)), was found to be the dominant wear process in these burs. Similarly, while grit dislodgment accounted for about 15% diamond loss (Fig. 14(a)), when machining zirconia with the fine burs, grit fracture (Fig. 14(b)), was found to be the dominant wear process. Several diamond grit were found to contain wear flats, where the grit edges had been worn producing small flat surfaces (Fig. 14(c)). Wear flat generation, or attritious wear, was also observed in burs used for porcelain machining, but it was not a common wear process with
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Fig. 9. SEM micrographs showing typical chipping damage around the groove top surfaces for porcelain cut with (a) an ultrafine bur, (b) a fine bur, and (c) a coarse bur.
either material. Abrasion damage to the nickel matrix was also observed (Fig. 14(b)).
4. Discussion In this study, we have compared the abrasive machining behavior of dental porcelain with a high-strength zirconia using dental burs of three different grit sizes. The results clearly show a distinct difference in response for the two materials, both with respect to the removal rates achieved during cutting and the basic removal mechanisms. The removal rates for the porcelain were larger by at least one order of magnitude than for zirconia. While the removal
Fig. 10. SEM micrographs showing no chipping damage around the groove top surfaces for zirconia cut with (a) an ultrafine bur, (b) a fine bur, and (c) a coarse bur.
rates for the porcelain were equivalent to that of Dicor, a machinable glass ceramic [6], the removal rate of zirconia was close to the rate obtained for InCeram alumina [7]. These four ceramics seem to show a correlation between the removal rate and hardness. However, a more systematic analysis is needed before an authoritative statement can be made. Although the removal rates of porcelain and zirconia are significantly different, the surface roughness parameters are almost indistinguishable when the same bur grit size is used. This material-independent roughness behavior is consistent with the previous results for glass-ceramics [6] and glass-infiltrated alumina [7], and with a Monte Carlo simulation of surface roughness generated by abrasive machining
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Fig. 11. Average chipping width around the groove surfaces in porcelain cut with coarse, fine, and ultrafine diamond burs. Each data point is the mean value from three repeat cuts with three different burs under the same machining conditions; the error bars represent ± one standard deviation for the cuts.
[29]. In that study, surface roughness was shown to be primarily sensitive to the grit size, consistent with the present results. Diamond grit size plays an important role in abrasive machining. It is generally believed that higher removal rates can be achieved by coarser grit burs, and therefore, coarse burs are often used for gross tooth reduction [8]. The relationship between the removal rate and grit size, however, is not necessarily linear for all materials. While the removal rate for porcelain was higher by about 15% with coarse burs than with fine burs, the removal rate for zirconia increased by nearly 50% upon the use of coarse burs. The removal rate for machinable glass-ceramics did not always increase with the use of coarse burs [6]. However, coarse burs substantially increased the extent of chipping damage along the edges of the grooves cut in glass-ceramics [6], similar to the results on porcelain in the present study. The zirconia ceramic, unlike the porcelain, did not exhibit any visible edge chipping with any of the burs used in this study. Since edge retention at the margins and the achievement of close dimensional tolerances are important issues in obtaining high-quality restorations, one must balance the high-removal rates often associated with the use of coarse burs with the potential loss of surface/edge integrity. The use of coarse burs could also increase the propensity for generation of subsurface cracks with concomitant strength degradation [20]. The strength reduction due to machining-induced damage is, however, material-dependent. The yttria-stabilized tetragonal zirconia is less sensitive to machining damage compared to many other polycrystalline ceramics [26]. The effect of grit size on removal rate and edge chipping is not necessarily a simple phenomenon. Usually, an
increase in the grit size is accompanied by a reduction in the number of diamond particles in dental burs [6]. Therefore, with the same load is applied to the bur, the grit load can be much larger with a coarser bur. The higher grit load causes an increased grit penetration into the ceramic workpiece resulting in a higher removal rate and generation of surface and subsurface cracks. Both the removal rate and the extent of chipping damage depend on the same properties that control deformation and fracture of the ceramics, i.e. hardness and toughness [30], that also control the mode of material removal, i.e. brittle fracture versus plastic deformation [20]. The material removal mechanism in abrasive machining of porcelain was dominated by brittle fracture, as seen by the pits with sharp edges on the machined surfaces (Fig. 5) and the fragmented appearance of the machining debris (Fig. 3). However, the removal mechanisms for zirconia were primarily plastic deformation and microcutting, as evidenced by the smooth furrows on the machined surfaces (Fig. 6) and the ribbon-like shape of the machining debris (Fig. 4). The difference in the behavior of these two materials with respect to the predominance of brittle fracture or plastic deformation is consistent with Hertzian contact studies [27]. For the porcelain, the response to indentation with a tungsten carbide sphere was classical brittle fracture, i.e. formation of a well defined cone crack [27]. The response for zirconia, however, was primarily plastic or quasi-plastic, i.e. evolution of a diffuse subsurface zone of microscopic shear cracks [27]. The brittle/ductile response of these two materials to indentation can also be predicted by using a brittleness index proposed by Rhee et al. [31], derived from the ratio of the contact load needed for initiation of subsurface plastic
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Fig. 12. The volumetric removal rate as a function of total machining time in prolonged cutting tests for (a) porcelain and (b) zirconia. Diamond burs of three different grit sizes (ultrafine, fine, and course) were used in the tests with porcelain, while only a fine bur was used with zirconia. The burs were ultrasonically cleaned several times during the test series.
yield to the threshold load for the formation of ring cracks. The distinction between the brittle/ductile behavior of these two materials can also be predicted by using a similar brittleness index proposed by Quinn and Quinn [32], related to the ratio of deformation energy to fracture energy. The disposable or single-patient diamond burs employed in the present study are chosen by the clinicians to minimize cross-contamination risks of bloodborne pathogens [33]. It is likely that such burs would be used no more than a few tens of seconds before being discarded. However, our results indicate that these single-patient burs can be used for several minutes without a significant loss of cutting efficiency.
The cutting efficiency is measured by the reduction in the removal rate as a function of cutting time. The decrease in the removal rate as a function of cutting time is generally due to bur loading (attachment of machining debris to the bur) and wear of the diamond particles. The influence of bur loading in our study, however, was minimal since ultrasonic cleaning of the burs prior to the fourth cuts did not significantly increase the removal rate (Fig. 2). Wear of the burs occurred through grit fracture, grit pullout, wear flat generation or attritious wear, and matrix damage. These results are consistent with the observed wear mechanisms previously discussed for dental cutting
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Fig. 13. SEM micrographs of the burs after prolonged cutting tests on porcelain: (a) grit dislodgment on the ultrafine bur, (b) grit fracture on the fine bur, and (c) grit fracture on the coarse bur.
of glass-infiltrated alumina [7]. The observed grit fracture suggests that a friable (self-sharpening) diamond had been used in the burs, which may explain the fairly high-cutting efficiency of the burs in prolonged tests. The wear mechanisms in diamond burs can also be influenced by the grit size. It is generally accepted that finer diamond particles are stronger than the larger ones due to the lower flaw population as the original flaws in the large particles are eliminated by fracture when finer grit is made by crushing the coarse diamond particles [34]. Therefore, less grit fracture is expected for finer burs. However, fine burs are more prone to matrix damage since the space available for debris removal is smaller for the fine burs. The machin-
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Fig. 14. SEM micrographs of a fine bur prolonged cutting tests on zirconia showing different types of wear and damage: (a) grit dislodgment, (b) grit fracture and matrix abrasion, and (c) wear flat generation.
ing debris could cause abrasion damage on the metal matrix used to hold the diamond particles on the bur, weakening the bond between the diamond particles and the matrix, and resulting in grit loss. Therefore, grit loss becomes a more dominant wear process for finer burs.
5. Conclusions In this study, we investigated the machining characteristics of feldspathic porcelain and tetragonal zirconia with
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a dental handpiece and diamond burs of varying grit sizes under simulated clinical cutting conditions. The following conclusions are drawn from the results: (1) As the diamond grit size was increased from ultrafine (10 m) to fine (40 m) and coarse (151 m) the removal rate and the resulting surface roughness for both materials increased substantially. (2) The removal rate for porcelain was much higher than for zirconia for all grit sizes. (3) The porcelain sustained severe edge chipping with both coarse and fine burs, but edge chipping was essentially absent with the ultrafine burs. The zirconia was free of edge chipping for all grit sizes. (4) The surface roughness increased with increasing grit size for both porcelain and zirconia, and it was materialindependent. (5) The mechanisms of material removal determined through microscopic examination of the machined surfaces and the machining debris on the burs were found to consist of a combination of ductile and brittle-type chip formation processes. The occurrence of brittle fracture increased as the diamond grit size was increased. While the material removal process in the porcelain was dominated by brittle fracture, the removal process in zirconia was dominated by ductile cutting. (6) Four wear processes were identified on the burs in prolonged cutting tests: grit microfracture, grit pullout, wear flat generation, and matrix abrasion.
Acknowledgements We acknowledge the generous supply of materials from Norton St. Gobain and Vita Zhanfabrik. This project was supported by NIDCR Program Project Grant No. P01 DE 10976. We are grateful to Professors E.D. Rekow and V.P. Thompson of the New York University College of Dentistry for their overall guidance of this program.
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